As is known, double-cylinder circular knitting machines for hosiery generally comprise a lower needle cylinder which has a vertical axis and an upper needle cylinder which is arranged above and coaxially with respect to the lower needle cylinder, said cylinders being actuatable rigidly with each other with a rotary motion about the common axis.
A plurality of axial slots are formed on the lateral surface of the lower needle cylinder and on the lateral surface of the upper needle cylinder. The axial slots of the upper needle cylinder are aligned with the axial slots of the lower needle cylinder. A selector and a slider are generally accommodated in each of the axial slots of the lower needle cylinder, starting from the bottom, while each of the axial slots of the upper needle cylinder accommodates a slider. Between the two needle cylinders, i.e., in the knitting region, in each of the axial slots there is a needle which is provided with two tips or heads, respectively an upper head and a lower head; depending on whether one wishes to provide plain stitches or purl stitches, said needle is moved into the lower needle cylinder so that its knits with its upper tip or into the upper needle cylinder so that its knits with its lower tip.
Since the needle does not have a heel, it is actuated by means of the slider arranged in the lower needle cylinder or by means of the slider arranged in the upper needle cylinder, depending on whether it has to form plain or purl stitches.
The sliders currently used in double-cylinder circular knitting machines for hosiery are constituted generally by an elongated laminar body, which has a first longitudinal side designed to rest on the bottom of the axial slot formed on the lateral surface of the lower needle cylinder or on the lateral surface of the upper needle cylinder.
Said sliders are further provided with two heels, which are mutually spaced along the longitudinal extension of the slider and protrude transversely from a second longitudinal side of the slider which lies opposite the first side.
These heels are used to cause the movement of the slider along the corresponding axial slot of the lower or upper needle cylinder so as to cause the actuation of the needle associated with said slider in the various types of knitting of the machine or to transfer the needle from one needle cylinder to the other.
The slider is further provided, on its first longitudinal side, i.e., on its side directed toward the bottom of the axial slot within which it is accommodated, with a hook-shaped tab, which can engage the lower head of the needle or the upper head depending on whether the slider is in the lower needle cylinder or in the upper needle cylinder.
Around the lateral surface of the lower needle cylinder and around the lateral surface of the upper needle cylinder there are a plurality of slider actuation cams, which define a series of paths with which the heels of the sliders engage when the needle cylinders are actuated with a rotary motion about their axis with respect to said cams. The paths defined by the cams are shaped so as to cause the movement of the sliders along the axial slots of the needle cylinders in which they are accommodated and consequently cause the actuation of the needles that are associated therewith.
In currently commercially available double-cylinder circular knitting machines for hosiery, many of the cams that define the paths for the heels of the sliders are provided so that they can move along a radial direction with respect to the needle cylinders, so that they can be transferred from an active position, in which they are close to the needle cylinders so that they are engaged by the heels of the sliders, to an inactive position, in which they are spaced from the needle cylinders so as to not interfere with the heels of the sliders, or vice versa, in order to allow to vary the paths for the heels of the sliders and consequently vary the types of knitting that the machine can perform.
The presence of these movable cams and of the corresponding actuators, which is necessary in order to produce the various kinds of knitting, has the problem of increasing considerably the complexity of the structure of the entire machine.
Moreover, the presence of these movable cams forces the provision, on board the machine, of an appropriately provided actuation program, which intervenes if an accidental stop of the machine occurs due to failure of the electric power supply and restores the correct position of the movable cams before knitting resumes, since if the machine were to restart without first restoring the correct position of the movable cams the heels of the sliders might break.
In practice, the presence of these movable cams makes it necessary to provide the machine with electronic programs which store the position of the movable cams when the electric power supply is interrupted, and this constitutes a further complication in the production of the machine.
Similar but smaller problems can be observed in single-cylinder circular knitting machines for hosiery, i.e., knitting machines provided with a single needle cylinder, which use needles and optionally sub-needles provided with a heel which protrudes from the corresponding axial slot and can engage paths defined by needle and sub-needle actuation cams, which are arranged around the needle cylinder; some of said cams are movable along a radial direction with respect to the needle cylinder so that they can be moved from an active position, in which they are close to the needle cylinder so as to be engaged by the heels of the needles and/or of the sub-needles, to an inactive position, in which they are spaced from the needle cylinder so as to not interfere with the heels of the needles and/or sub-needles, or vice versa, in order to allow to vary the paths for the heels of the needles and accordingly vary the types of knitting that the machine can perform.